We have been investigating the structural requirements in neurophysins (NP's) for hormone binding, self-association, and antibody recognition. The principal approach during this period has been to determine the alteration in these properties in NP's that have been derivatized. Bovine NP derivatized by trypsin digestion and photo-affinity labelling of the hormone binding site were analyzed by affinity chromatography and radioimmunoassay. These data were correlated with radioimmunoassay data for disulfide-broken NP's and native NP's with different sequences. Overall, the results showed that NP hormone binding and self-association are critically dependent upon a structurally intact protein molecule. In contrast, antibody recognition is lost only in NP derivatives in which disulfide bonds are broken.